Such a rotor includes:
a longitudinal rotation axis;
a magnetic circuit which forms poles which project radially outwardly, extend longitudinally and are spaced out angularly about the axis, each pole including a polar core and ending near the outside in a projecting pole piece whose edges project angularly on either side of the polar cores;
exciter windings between the polar cores;
damper bars made of a substance which is a good conductor of electricity and extending longitudinally without discontinuity along the whole length of the poles, some of these bars being interpolar damper bars disposed between the poles; and
two conductor rings disposed coaxially with the rotor at respective ends thereof and electrically connected to respective ends of each damper bar to form a damper screen of the squirrel cage type with these bars. It is applicable advantageously to low-speed alternators, e.g. in a submerged hydro-electric turbo-alternator of the bulb type, herein called, for short, a bulb unit.
Such a unit provides electric power at at least 5 MVA, e.g. 40 MVA. It is located in a bulb i.e. in a sealed profiled chamber with a generally horizontal axis, said bulb itself being disposed in the axis of a water duct. It includes a turbine whose moving blades intercept the liquid flow which flows past the bulb on all sides thereby driving the rotor of an alternator disposed inside the bulb.
The water duct, the bulb, the turbine and the alternator are aligned along the same axis.
The magnetic circuit of an alternator's stator or, in short, the stator magnetic circuit is constituted by a ring whose radially internal portion has grooves which contain the bars of the stator winding. The diameter of said circuit is always limited, in particular, in the case of a bulb unit, by the maximum diameter which can be allowed for the bulb which contains it, it being generally impossible to allocate a diameter of more than 5 to 6 meters to the space occupied by the rotor.
The mechanically strong part of the ring of the stator magnetic circuit which is confined to the zone where the grooves are not cut is therefore radially relatively thin and therefore it is comparatively much less rigid than in a conventional alternator. Since the rotation speed of the bulbs is low (e.g. less than 100 or 120 r.p.m.) these alternators have a large number of poles which being installed on a limited diameter, are spaced around the periphery at a relatively small polar pitch (e.g. 20 to 30 cm).
To obtain a voltage whose waveform is substantially sinusoidal at the terminals of the machine, the winding of the armature must have a non-integer number of grooves per pole and per phase. Such a winding is very rich in space harmonics due to armature reaction, in particular in sub-harmonics whose space period extends over several pairs of poles.
Further, the small diameter of the units leads to very much smaller air gaps being adopted than with conventional machines so as to be able to maintain acceptable induction in the air gap without increasing the excitation current too much, taking into account the small polar pitch of these machines. The result of this is that the notch harmonics are greater in low-speed machines such as bulb machines than in larger hydroelectric alternators.
In these conditions, due to the fact that when the stator magnetic circuit is thin and therefore does not withstand radial bending to any great extent, there is a high risk of vibration due to the effect of the notch harmonics or of the numerous armature reaction harmonics.
In this respect, it should be noted that the space pitch and the rotation speed of each armature reaction harmonic are very different from one another with respect to the rotor. The frequency of the notch harmonics with respect to the rotor is high (8 to 12 times the power-line frequency) and its space pitch may extend over several poles.
From the above, it ensues that when it is sought to obtain high electric power at the output of a low-speed alternator, the vibrations of the ring of the stator magnetic circuit may reach a detrimental level. It is known that in conventional alternators, the forces of electromagnetic origin to which said ring is subjected can be limited by installing a damping winding disposed at the periphery of the rotor. Said winding is generally of the squirrel cage type and is constituted by longitudinal conductor bars, i.e. bars parallel to the axis and connecting two conductor rings disposed at respective ends of the rotor. Some of these bars are polar bars, i.e. they are sunk in the outer zone of the projecting pole pieces. Others may be interpolar bars, i.e. they may be disposed between the poles. For example, an interpolar bar 15 is shown in FIG. 5 of French Pat. No. 1 475 482 (Moskovsky Energetichesky Institut).
The effect of centrifugal forces on such known bars is countered by auxiliary means, e.g. resin interposed between the interpolar bar and the air gap of the alternator, thereby reducing the efficiency of the damping means.
In particular, it ensues from the above that known dispositions do not allow the winding to damp efficiently those high-frequency harmonics with large space pitches that are encountered in low-speed alternators with narrow air gaps, and this may cause the vibration level to be too high.
Preferred embodiments of the present invention provide a rotor with a damping screen for an alternator with projecting poles having interpolar damping bars to allow high-frequency flux harmonics with large space pitches to be damped in a particularly efficient way.